1. Field of the Invention
The present invention relates to a hydraulic control device for an automatic transmission for a vehicle such as an automobile, and more particularly, to a hydraulic control device for such an automatic transmission having two parallelly arranged input clutches which are selectively engaged or disengaged in providing a plurality of speed stages.
2. Description of the Prior Art
In Japanese patent application No. 62-195471 filed on Aug. 5, 1987 and laid open to public on Feb. 13, 1989, two of the inventors of the present invention have proposed an automatic transmission for a vehicle such as an automobile in which a speed stage shifting mechanism includes two parallelly arranged input clutches and is adapted so that a first one of the two input clutches is engaged when it provides a first speed stage, a second speed stage and a third speed stage, whereas a second one of the two input clutches is engaged when it provides the third speed stage and a fourth speed stage. However, in this speed stage shifting mechanism the engagement of said first input clutch is not essential for setting up said third speed stage unless engine brake availability is required since a one way is provided in parallel with said first input clutch, said one way clutch being adapted to be automatically engaged when the automatic transmission operates in a normal engine driving condition. On the other hand, the engagement of said first input clutch is essential for setting up said second speed stage in which said second input clutch is disengaged.
In the above-mentioned automatic transmission, assuming that the vehicle is running with the automatic transmission being set up in the fourth speed stage, when the automatic transmission is shifted down from the fourth speed stage to the second speed stage by a deep stepping down of the accelerator pedal, that is, the so-called kickdown, said first input clutch needs to be engaged relatively quickly so that there occurs no moment of virtual disconnection of the transmission in the transition from said second clutch engagement to said first clutch engagement since otherwise the engine will rev up and a substantial speed shift shock will follow it.
On the other hand, there are two possibilities for the transmission being shifted down from the fourth speed stage to the third speed stage. In a first possibility, the automatic transmission is shifted down from the fourth speed stage to the third speed stage by a stepping down of the accelerator pedal, that is, the so-called kick down, and in a second possibility the transmission is shifted down from the fourth speed stage to the third speed stage by the prohibition of the overdrive stage, provided that the fourth speed stage is an overdrive stage as mostly so in the modern automobiles and there is provided an overdrive selection means such as an overdrive cutoff button.
When the transmission is shifted down from the fourth speed stage to the third speed stage in the above-mentioned type automatic transmission, it is desired that, when the downshifting is due to the kickdown, said first input clutch is engaged relatively slowly as compared in the downshifting due to the overdrive prohibition, or in other words, when the downshifting is due to the overdrive prohibition, said first clutch is engaged relatively quickly as compared in the downshifting due to the kickdown, because, when the automatic transmission is shifted down from the fourth speed stage to the third speed stage by the kickdown, after a reaction brake (in fact brake B.sub.1 described hereinunder) was disengaged, engine rotational speed rises and the above-mentioned one way clutch (in fact one way clutch F.sub.1 described hereinunder) is automatically put into engagement, and therefore said first clutch may be slowly engaged or rather should be slowly engaged not to cause an abrupt engine braking, while in the downshifting due to the overdrive prohibition, since the accelerator pedal will not be stepped on, engine rotational speed will not rise to that said one way clutch is swiftly engaged, and therefore, if the engagement of the first clutch is delayed, the vehicle will run in an inertial condition as if it is running with the manual shift lever set to the neutral position. Such an inertial running condition, when it occurs without a positive instruction by the driver, will give the driver a feeling of danger of loss of engine braking.
Thus, it is noted that in the automatic transmission of the above-mentioned type the speed of engagement of said first clutch should be variably controlled according to the stepping on condition of the accelerator pedal or, more fundamentally, the output power of the engine.
The above-mentioned first input clutch is disengaged when the transmission is shifted from the third speed stage to the fourth speed stage. Further, there is another possibility for said first input clutch being disengaged during running of the vehicle. That is a shifting of the manual shift lever from the D range position to the neutral position. When said first clutch is disengaged in the upshifting from the third speed stage to the fourth speed stage, it is desirable that the disengagement is accomplished quickly. However, when the manual shift lever was shifted from the D range position to the neutral position while the vehicle is running under the D range in the third speed stage, it is desirable that the disengagement of said first clutch is slightly delayed in order to avoid any substantial shock to occur.